Global satellite communication systems have been designed which provide worldwide service to communication devices located on or near the ground. Many of these systems use "cross-links" between satellites to route data from one part of the world to another. In order for two satellites to communicate directly, the satellites must be in line-of-sight ("LOS") of each other.
To provide a fully-connected network of non-geosynchronous satellites, the communication system would need to deploy a relatively large number of satellites so that enough satellites encircle the earth to provide LOS between satellites. A large number of satellites increases the start-up and operational costs of the system. For example, one major start-up cost is the launch cost which is driven by satellite weight and number of satellites deployed. Systems are deployed a few satellites at a time. Therefore, a fully-connected satellite network is not possible during early stages of deployment because not enough satellites are in orbit to provide interconnectivity. This leads to another major system cost, which is the opportunity cost of lost market share and inability to generate substantial operations revenue early in the deployment phase of a satellite system.
In many instances, satellite services might not be available for communication due to prohibitive cost, insufficient transmission power, or other limitations. Such systems must rely on terrestrial RF links or hardwired links in order to communicate. Where RF or hardwired links do not exist, rapid communication also does not exist.
What are needed are an apparatus and method that enables communication services using a constellation having a smaller number of satellites or even no satellites. Further needed are an apparatus and method where communication devices and/or satellites with no LOS or hardwired connections can communicate with each other without any significant delays.
A ground device which communicates with a satellite must be specially adapted because of the proximity of the satellite to the ground device. For example, many ground devices have directional antennas which must be oriented toward a particular satellite. Where the satellite is non-geostationary, the antenna must track the satellite during the time that the ground device wants to receive signals. Some ground devices receive signals from multiple satellites and/or other sources. These multi-receiver ground devices must have dedicated directional antennas and, where necessary, tracking devices for each satellite or other signal source. For example, a ground device might be adapted to receive signals from two geosynchronous satellites, each satellite providing a different communication service. If the user of such a ground device wants to receive signals from a third source, the user must physically modify the ground device hardware or purchase a separate device which enables reception of signals from the third source. Thus, existing multi-receiver ground devices are inflexible and prone to being insufficient or becoming obsolete.
Another feature of satellite-to-ground communications is that the satellite and ground device must transmit signals at relatively high power in order for the signals to be received across the great distance between the satellite and ground device. Obstructions and low elevation angles between the ground device and the satellite and/or other signal source also mandate high-power transmissions and/or lack of high-quality signal reception. Besides high-power transmitters, the satellite and ground device also must have sensitive receivers in order to receive the signals from each other. Ground devices, thus, have many equipment requirements in order to communicate with one or more satellites or other signal sources. Because of these equipment requirements, such ground devices are typically very expensive when compared with the cost of equipment which communicates exclusively with terrestrial signal sources.
What are needed are relatively-inexpensive ground device for communication with one or more satellites and/or other signal sources and a method of operating such a device. Also needed are a flexible ground device and method of operation where the ground device can receive information from additional signal sources without modifying the physical hardware of the ground device. Also needed are an apparatus and method to enable operation of such inexpensive and flexible ground devices. Further needed are a method and apparatus which enable a ground device to better communicate with a satellite and/or signal source which transmits at an insufficient power level, is located at low elevation angle, or is located behind an obstruction relative to the ground device.
Currently, terrestrial cellular base stations and proposed communication satellites provide cellular communications to ground devices. The cellular beams from those base stations and satellites typically have precise geometries on the surface of the earth which enable non-adjacent beams to reuse allocated spectrum. Especially with respect to existing terrestrial base stations, each cellular beam requires dedicated hardware and cell geometries can only be modified by physically changing the characteristics of the hardware. Thus, altering cell geometries to respond in real-time to traffic loading is not a practical solution in such systems.
What are needed are a method and apparatus that enables high definition and flexible cell geometries without the need for hardware modifications.
Communication satellites also are used in sensing applications. For example, satellites are used to monitor weather or other physical phenomenon which occur proximate to the surface of the earth. The ability of such sensing satellites to observe phenomenon to a high-level of accuracy is limited by the distance of the satellites from the phenomenon and/or obstructions which exist between the satellite and the phenomenon. More expensive and heavier sensing equipment can be used to enhance accuracy, but only to the limit of current technology.
What are further needed are a method and apparatus for sensing physical phenomenon which provide more accurate data than is possible with sensing satellites.